Technical Abstract:
Previous ARS-NRC studies developed methodologies, and showed the value of high-frequency monitoring of unsaturated zone water contents and piezometric levels to estimate infiltration and ground-water recharge. This study tested those methodologies through comparison with measured ground-water recharge in a closed system using a highly monitored lysimeter (i.e., 14 by 20 by 3 meters). Specifically, near-continuous water content, water-table elevation, and meteorological data were collected to estimate infiltration and ground-water recharge and their attendant uncertainties. These highly-detailed monitoring data were evaluated to capture individual recharge-event characteristics (i.e., infiltration, drainage, and evapotranspiration) and estimation of hydraulic parameters. We used three methods to calculate ground-water recharge, 1) drainage estimates from real-time, near-continuous water content measurements, 2) numerical modeling using the HYDRUS-2D and the PNNL Water Balance models, and 3) seasonal steady state averages of rainfall and evapotranspiration. Results were compared to calculations of ground-water recharge from water table height in the lysimeter using an estimate of specific yield. Use of water content measurements gave low estimates of net ground-water drainage compared to those from ground-water level. Difficulty in quantifying actual evapotranspiration contributed to uncertainties in all three methods.